1. Field of the Invention
This invention relates to the display driving method and drive that can have a high density and fine image by constituting one dot of input signal with plural picture elements and displaying the half tone by way of error diffusion in unit of pixel.
2. Description of the Prior Art
Recently PDP (Plasma Display) has been attracting a great deal of public attention as a thin, light-weighted display device. Totally different from the conventional CRT drive system, the drive method of this PDP is a direct drive by means of digitalized image input signal. Consequently, the luminance and tone of the light emitted from the panel face depends on the bit number of the signal to be processed.
PDP may be divided into two types: AC and DC types whose basic characteristics are different from each other. The DC drive type PDP has reportedly improved the luminance and service life which had been one of the longstanding questions. This type of PDP is therefore progressing toward its commercial use.
AC type features satisfactory characteristics as far as the luminance and durability is concerned. As for the tonal display, maximum 64 tones only have reportedly been displayed at the level of trial production.
It is however proposed to adopt in future a technique for 256 tones by address/display separate type drive method (ADS subfield method).
FIGS. 1(a) and (b) are the drive sequence and drive waveform of the PDP used in this method.
In FIG. 1(a), one frame consists of 8 subfields whose relative ratios of luminance are 1, 2, 4, 8, 16, 32, 64 and 128 respectively. Combination of these 8 luminances enables a display in 256 tones. The respective subfields are composed of the address duration that writes in one screen of refreshed data and the sustaining duration that decides the luminance level of the corresponding fields. In the address duration, first wall charge is formed initially at each pixel simultaneously over all the screens for display. The brightness of the subfield is proportional to the number of the sustaining pulse to be set to predetermined luminance. Two hundred and fifty-six tones display is thus realized.
Since said address duration is constant irrespectively of the length of the sustaining duration, the more the number of tones in such an AC drive method, the more the number of bits of the address duration is as the preparation time for lighting up and making the panel luminescent within one frame of duration increases. The sustaining duration as light emitting duration becomes thus relatively short thereby reducing the maximum luminance.
Because the luminance and tone of the light emitted from the panel face depends upon the number of bits of the signal to be processed, increased number of the bits of the signal improves the picture quality, but decreases the emission luminance. If conversely the number of the bits of the signal to be processed is decreased, the emission luminance increases but it decreases the tone to be displayed, causing thus the degradation of the picture quality.
The error diffusion intended to minimize the color depth difference between the input signal and emission luminance rendering the number of the bits of the output drive signal smaller than that of the input signal, is a process to express false tone used when the maximal shade of color is desired to be manifested with lesser tone.
The basic way of thinking for the error diffusion is as follows.
When the image signal is converted from analog to digital, or the digital image signal is converted into that of lower bit number, we should note that the original image signals include an intermediate luminance that cannot be represented by any number of bits of the digital signals thus converted. There arises therefore an error between the original image signal and the image signal as converted, which will degrade the picture quality. The error produced from the conversion is distributed and added (diffused) to surrounding pixels that are spatially and temporally neighboring to each other to display falsely (illusorily) the half tone thereby suppressing the degradation of the picture quality. In general, the error is so distributed and added that the total sum of the errors should be equal to those detected against the image signal, before conversion, of at least one of the neighboring pixels processed after the pixels with error detected. The neighboring pixels processed after the pixels with error detected mean, in a normal display device, the rightneighboring, under-neighboring and right/underneighboring pixels spatially, and those at the same position in the following picture temporarily.
FIG. 2 shows a conventional, general error diffusion circuit, where an image signal with the original picture elements or pixels Ai, j of n (8, for example) bits is input into an image signal input terminal 30. This image signal is processed in a vertical adder 31 and horizontal adder 32, its bit number being reduced to m (4, for example). After passing through an image output terminal 34 and PDP drive circuit, it makes the PDP luminescent.
The ROM 38 in the error detect circuit 35 stores in memory the data of the signal after the conversion (reduction) of the bit number by the bit conversion circuit 33 in a corresponding fashion to the pixel signal before the bit number conversion. When the signal from the foregoing horizontal adder 32 enters the ROM 38, the data outputs of the corresponding signal after the bit number conversion. The adder 39 outputs as an error the difference between the signal from the ROM 38 and the signal from the horizontal adder 32.
The error signal as output from the adder 39 is weighted by predetermined coefficient at the error weight circuits 40 and 41 to get an error detect output. This error detect output is added to the foregoing vertical adder 31 through the intermediary of an h-line delay circuit 36 that outputs a reproduced error E(i, j-1) that has occurred in the pixel by h lines behind the original pixel A(i, j), for example, by one line, and at the same time, it is added to the foregoing horizontal adder 32 through the intermediary of a d-dot delay circuit 37 that outputs the reproduced error E(i-1, j) that has occurred in the pixel by d dots behind the original pixel A(i, j), for example, by one dot.
That is, the reproduced error E(i, j) as detected from the original pixel A(i, j) is added to pixel signal A(i, j+1), by one line behind, through the intermediary of the h-line delay circuit 36, and further to the pixel signal A(i+1, j), by one dot behind, through the d-dot delay circuit 37.
Similarly, the vertical adder 31 adds to the original pixel A(i, j) the reproduced error E(i, j-1) of the pixel A(i, j-1) which is by one line behind, while the horizontal adder 32 adds to the same original pixel the reproduced error E(i-1, j) of the pixel A(i-1, j) which is by one dot behind.
In general, the coefficients at the error weight circuits 40 and 41 shall be so set that the total sum of all these coefficients be one (1).
As a result, 16-tone signal represented by 4 bits is output from the output terminal 34 of the bit conversion circuit 33, and correspondingly the emission luminance level becomes 16-tone as shown by the solid line in FIG. 4. When the drive signal of the original pixel as represented by 8 bits is converted into 4-bit signal at the bit conversion circuit 33, eliminating the lower 4 bits allows in general to give 16 tones with 0 to 15 converted into 9, 16 to 31 into 16, 32 to 47 into 32, . . . and 240 to 255 into 240. After this conversion, we get such stepwise drive signal and emission luminance level as shown by the solid lines in FIG. 4.
Since, however, human eyes recognize the emission luminance of surrounding pixels as spatially and temporally averaged, the display screen after the error diffusion is perceived by human eyes as displaying an intermediate tone that cannot be represented by the 16 tones due to the averaged emission luminance. That is, even though the emission luminance cannot but be represented skippingly as shown by the solid lines in FIG. 4, it is recognized by human eyes as a corrective luminance line of y=x shown by the dotted lines.
The scale of the horizontal and vertical axes in FIG. 4 represent, as maximal value, the maximum 255 when the original pixel is represented by 8 bits.
The driving method as shown in FIG. 1(a) adopts 256 tones dividing one frame into 8 subfields. Increasing this number of tones reduces the emission luminance. If, conversely, the bit number of the signal to be processed is decreased composing one frame with 6 subfields as shown in FIG. 3(a), the emission luminance increases. If the same is done configuring one frame with 4 subfields as shown in FIG. 3(b), the increasing trend of emission luminance becomes greater.
Such half tone display technique as has been described was problematical in that it reduces the resolution and elicits particular patterns because the brightness is diffused in the respective directions of vertical, horizontal and time.